Present day electron beam ion sources operate with electron beam currents of less than 1 ampere. If commercially practical sources could be built for generating electron beams having multi-ampere outputs, numerous applications are known to exist for such high current sources. Most such applications require electron beams that operate on a steady state basis, or with very long pulses of current, and that also have low emittance. It is generally known that plasma generating cathode devices can readily yield multi-ampere currents of electrons, however, it is also generally known that such high current sources are characterized by having very high emittances. Thus, one skilled in the art of electron beam generators would not ordinarily look to plasma cathode sources for the production of electron beams that must be designed to have low emittance.
Currently available plasma cathode beam sources, due to their high emittance, have found very limited practical application, which is mostly limited to exciting molecules in powerful gas lasers. For most other current applications of electron beams, the beams are injected from guns that utilize either thermionic cathodes or photocathodes in which electrons are emitted from a surface and have energy spreads of 0.5 electron volts (eV) or less. Such limited energy spreads are relatively small, compared to the energy spreads of at least several eV for beams produced from plasma generators. Although such narrow energy spreads are desirable for many potential beam applications, the inherent space charge problems of currently used surface emitted electron sources pose severe limitations on the total steady state, or reasonably long pulsed, electron currents that can be extracted from such surface electron emitters. Other types of surface electron emitters, such as semi-conductor photoemitters, have a further inherent limitation in that they can only generate very short pulses of output beam current, when operated to produce high current beam outputs.